The present work describes the development of a highly sensitive amperometric sensor based on a simple and efficient carbon paste electrode modified with a new redox "mediator", 4-nitrophthalonitrile, toward detection of cysteine (CySH) in neutral medium at an applied potential of 0.33 V vs. Ag/AgCl. The sensor showed its best performance in 0.1 mol L-1 phosphate buffer solution, at pH 7.0. After optimizing the operational conditions, the sensor provided a linear response range for CySH from 0.8 up to 13.2 mu mol L-1 with sensitivity, detection and quantification limits of 37 nA L mu mol(-1), 0.25 mu mol L-1 and 0.80 mu mol L-1, respectively. The sensitivity of the proposed sensor was much higher when compared to other sensors described in the literature and showed a stable response for at least 80 successive determinations. The repeatability of the measurements with the sensor and the preparation of a series of electrodes, evaluated in terms of relative standard deviation, were 4.1 and 5.0%, respectively, for n = 10. The developed sensor was applied for cysteine determination in food supplement samples and the results were statistically the same with those obtained by the comparative method described in the literature at a confidence level of 95%. (c) 2007 Elsevier B.V. All rights reserved.